Alternating current power source for welding

ABSTRACT

A AC power source for welding is configured to receive three phase power and to provide single phase current to a welding load. The power source includes a three phase transformer with three center tapped secondary windings, a center tapped output inductor, and suitably triggered thyristors between the secondaries and the output inductor.

BACKGROUND OF THE INVENTION

The invention concerns an alternating current power source for welding,in particular for TIG and electrode welding, for generating analternating current from a three-phase current.

In the welding of metals, which form nonconducting and/or high-meltingoxides, by the TIG process, it is necessary for the purpose of removingthe oxide skin to operate either with reversed polarity (positive poleat the electrode), which is possible, however, only with low currentintensities in view of the excessive generation of heat at the weldingtorch, or with an alternating current, which corresponds to the currentpractice.

In electrode welding with a direct current, blow effects are frequentlyencountered, which render the control of the arc impossible. Here,again, an alternating current must be used.

According to the prior art, welding transformers are used for thispurpose, which are connected on the primary side to two phases of thethree-phase supply network and wherein the welding current is regulatedby means of scatter yoke cores, transductors or thyristors withphase-gating control.

Power sources of this type have the disadvantage of a high powerrequirement, which in the case of the welding currents usually requiredrapidly exceeds the available connected load. Another disadvantage isthe asymmetrical loading of the three-phase supply network.

In order to eliminate these disadvantages, more recently power sourceshave been developed, which are connected to all three phases of thenetwork and contain a three-phase bridge rectifier, together with asubsequent thyristor inverter or transistor inverter. As againstconventional transformers with thyristors, such power sources arenaturally complicated and expensive.

SUMMARY OF THE INVENTION

It is therefore the object of the invention to provide at acomparatively low cost an alternating current power source which loadsall three phases of the three-phase supply network symmetrically.

According to the invention, this object is achieved by a circuit for anAC power source for welding, in particular for TIG (tungsten inert gas)and electrode welding, which generates an alternating current from athree-phase current and includes a three-phase transformer, 6 electronicswitches, preferably thyristors and a throttle with center tapping. Thethree secondary windings of the transformer have center taps, which whenswitched together form a welding current connection. Threeunidirectional windings starts lead to the cathodes of three thyristors,the anodes whereof are combined and lead to a winding start of thethrottle. The three unidirectional winding ends lead to the anodes ofthree thyristors, the cathodes whereof are combined and lead to theother winding end of the throttle. The center tap of the throttle formsthe second welding current connection. The thyristors are triggeredsequentially so that the actuation of the thyristor at the start of thewinding of the first transformer leg is followed by the actuation of thethyristor at the winding end of the second leg and subsequently by theactuation of the thyristor at the winding start of the third leg,followed by the actuation of the thyristor at the winding end of thefirst leg, etc.. The thyristor act as welding current control elementsby means of phase shift control and the phase angle is limited to max120° in order to prevent internal power source short circuits betweenthe phases. An alternating current is generated in this manner on thesecondary side, and all three phases of the network are uniformly loadedon the primary side. Thus, the power source essentially consists of athree-phase transformer, an inductor and six expensive thyristors with alow off-state voltage for system frequency applications and with acircuit for their sequential triggering.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the drawing and is described in moredetail hereinbelow.

FIG. 1 shows the circuit diagram of the alternating current power sourceaccording to the invention.

FIG. 2 shows the output waveform from the power source according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

For a better comprehension of the operation of the circuit layout, letus consider only the upper part of the transformer winding 1, 2, 3 onthe secondary side and of the thyristors, 4, 5, 6 as well as theinductor 10. This circuit segment shows a Y-connection. The same is alsotrue, if considered in itself, for the lower part of the circuit layoutwith the reversely poled thyristors 7, 8, 9. If in the Y-connection thephase angle is raised to a maximum of 120° only, always only onethyristor is conducting. If the thyristors 4,5 and 6 are sequentiallytriggered, a direct voltage with a pulse frequency of 150 Hz isgenerated on the output side. If, in the circuit layout according to theinvention, the thyristors are triggered in the following sequence: 4 -8 - 6 - 7 - 5 - 9 - 4 - 8 . . . , an alternating current voltage isgenerated at 75 Hz on the output side, the variation of which is shownin FIG. 2. The inductor 10 serves to smooth the welding current, sinceit passes through it in the same direction. The negative half-waves flowfrom the winding start, the positive half-waves from the winding end, tothe center tap.

The advantage to be achieved by the invention thus consists of creatingby means of the circuit concept and the sequence an alternating currentpower source which is very simple in its configuration and, as it usesonly six thyristors and simple structural parts, inexpensive and whichloads the three-phase supply network symmetrically and reduces theon-phase.

I claim:
 1. An AC power source for welding, of the type configured to receive three phase power and to provide single phase current to a load having first and second terminals, comprising:a three-phase transformer having first, second and third center tapped secondary windings wherein each of said secondary winding has a first end, a second end, and a center tap; a center tap output indicator having a first end and a second end and a center tap; and first and second electronic switches in electrically operative association with said inductor and each of said secondary windings, said electronic switches disposed to conduct current selectively through said secondary windings and in the same flux-creating direction through said inductor regardless of the direction of current through the load.
 2. The power source of claim 1, wherein said secondary windings are connected in a star configuration and said center tap of each of said secondary windings is in electrically operative association with the first load terminal.
 3. The power source of claim 2, wherein said inductor is a center tapped inductor, and said center tap of said inductor is disposed in electrically operative association with the second load terminal.
 4. The power source of claim 3, wherein each of said first and second switches is a thyristor.
 5. The power source of claim 4, further comprising phase control means for sequentially rendering said thyristors conductive in a sequence which establishes current flow in said secondary windings in alternating flux-creating directions, whereby current flows in said first secondary winding prior to current flowing in said second secondary winding, current flows in said second secondary winding prior to current flowing in said third secondary winding, and current flows in said third secondary winding prior to current flowing in said first secondary winding.
 6. The power source of claim 5, wherein said phase control means includes means for controlling the conduction period of each thyristor wherein each conduction period is equal to one-quarter of the period of said three phase power.
 7. The power source of claim 5, wherein said phase control means includes means for controlling the length of time current flows though each of said secondary windings, wherein current flows through each of said secondary windings for an equal length of time.
 8. The power source of claim 1, wherein each of said first electronic switches is connected between said first end of each of said secondary windings and said first end of said center tap inductor, and further wherein each of said second electronic switches is connected between said second end of each of said secondary windings and said second end of said second tap inductor. 